DESCRIPTION: (Applicant's Description) The po transcript of the human c-myc gene is initiated over 600 bp upstream of the major Pl/P2 start sites, and its sequence corresponds to regulatory regions of the core promoter DNA. We hypothesize that this 5'-untranslated RNA sequence is a fundamental participant in the transcriptional regulation of c-myc and the ultimate determination of cellular phenotype. In preliminary experiments, we have determined that the PO RNA is capable of binding specific DNA sequences within the core Pl promoter and the first exon of c-myc. Furthermore, we have found that enforced expression of the sequence unique to the po transcript is associated with drastic, alterations in phenotype of malignant cells, including greater than 90 percents decrease in anchorage- independent growth in soft agar, and complete loss of the ability to form tumors in I immunodeficient mice. We propose to explore the potential therapeutic utility of this RNA sequence for modulating c-myc expression and interfering with proliferation, invasion, and metastasis of human breast cancer cells. The Specific Aims are: 1. Investigate the potential of the 5'-untranslated RNA sequence of the human c-myc PO transcript to regulate c-myc expression, by interacting with and blocking access to particular segments of the promoter / template DNA, or by specifically recruiting /sequestering individual polypeptide transcription factors. 2. Enforcibly express the 5'-untranslated sequence of the c-myc PO transcript in malignant and nontransformed human breast cells, to evaluate the potential therapeutic utility of this natural RNA sequence for modulating the proliferative and invasive capacity of the malignant cells. 3. Assess the degree to which artificial manipulation of the PO 5'-untranslated sequence alters the inherent tumorigenic, invasive, and metastatic potential of human breast carcinoma xenografts in immunocompromised mice.